Plastic optical materials are lightweight, hardly breakable, and excellent in dyeability as compared with optical materials made of inorganic materials such as glass. Therefore, plastic materials of various resins are widely used as optical materials for eyeglass lenses, camera lenses, and so on. In recent years, there has been an increased demand for higher performance of optical materials, particularly in terms of high transparency, high refractive index, low specific gravity, high heat resistance, and high impact resistance.
A polythiourethane-based compound is widely used as an optical material by virtue of its excellent optical features and excellent mechanical properties. A polythiourethane-based compound can be prepared by polymerization of a polythiol compound and an isocyanate compound. In such event, the physical properties of the polythiol compound have significant impacts on the physical properties of the polythiourethane-based compound to be prepared.
Lenses made from a polythiourethane compound are widely used by virtue of their high refractive index, light weight, and relatively high impact resistance. However, since the polythiourethane-based lenses have a lower Abbe number than that of glass lenses, the sharpness of the polythiourethane-based lenses is lower than that of the glass lenses, which results in a greater eye fatigue when the lenses are worn. Further, the polythiourethane-based lenses have a lower heat resistance than that of the glass lenses.
In recent years, efforts have been made to improve the optical properties thereof such as transparency and refractive index by way of increasing the purity of the raw materials or controlling the reaction in an attempt to obtain a transparency equivalent to that of glass.
For example, Korean Patent No. 1338568 discloses a process for preparing a polythiol compound by hydrolyzing an isothiouronium salt obtained by reacting a (poly) halogen compound or a (poly)alcohol compound with a thiourea, in which a colorless and transparent (poly)thiol compound is obtained as the content of calcium in the thiourea is 1.0% by weight or less. In addition, Korean Patent No. 1464942 discloses a process for preparing a polythiol compound for an optical material, which comprises reacting an epichlorohydrin compound with 2-mercaptoethanol, wherein the total amount of impurities contained in the epichlorohydrin compound is 0.45% by weight or less, and wherein the impurities consist of acrolein, allylchloride, 1,2-dichloropropane, 2,3-dichloropropene, 2-methyl-2-pentanol, 2-chloroaryl alcohol, cis-1,3-dichloropropene, trans-1,3-dichloropropene, 1,3-dichloroisopanol, 1,2,3-trichloropropane, and 2,3-dichloropropanol.